1. Field of the Invention
This invention relates to a method and an apparatus for measuring particles, and more particularly to a method and an apparatus for measuring the properties of particles in a fluid.
2. Description of the Prior Art
In the conventional apparatus of this type, a laser beam is directed into a fluid in a measurement cell containing the particles and the diameter of the particles is calculated from the intensity of scattered light from the particles, utilizing the fact that the intensity of the light scattered from the particles is a function of the particle diameter. Further processing is then conducted to obtain the number and density of the particles from the measured volume of the fluid.
The conventional apparatus will be explained with reference to FIG. 4. A laser beam emitted by a laser beam source 1 is converged on the measurement zone 4 within a measurement cell 3 by a lens 2. When one or more particles pass through the measurement zone, they scatter the laser light. The light scattered by the particles is condensed by a lens 5 and forms an image at a slit 6.
The light passing through the slit 6 is received by a photomultiplier 7, which converts it into an electric signal. The electric signal is amplified by a preamplifier 8 and is thereafter analyzed by an analog method or a photon-counting method using an analyzer 9 for analyzing the particle size distribution.
The relationship between the scattered light intensity and the particle diameter is generally determined experimentally using standard polystyrene latex mixed with water. However, it is known from the Mie scattering theory that the intensity of the light scattered by particles in a fluid is a function of the diameter of the particles, the refractive index of the particles and the refractive index of the fluid.
Therefore, any change in the refractive index of the fluid will modify the light scattering efficiency of the particles so that the intensity of the scattered light will differ even for one and the same particle. For concretely demonstrating this fact, the relationship between the refractive index of the fluid and the particle light scattering efficiency was investigated using a laser beam having a wavelength of 633 nm and particles having a diameter of 0.2 micrometer and a refractive index of 1.592, with the scattering efficiency at the index of refraction of 1.33 of the standard fluid
(water) being defined as 1. The results are shown in FIG. 5.
Conventional apparatuses using the laser beam scattering method for measuring fine particles in a fluid are not equipped with a means capable of correcting the measured intensity of the scattered light from the particles (or the measurement conditions etc.) on the basis of the scattering efficiency, which, as shown in FIG. 5, is dependent on the refractive index of the fluid. Up to now, therefore, the fact that the intensity of the light scattered from a particle changes when the fluid (refractive index of the fluid) changes has made it impossible to conduct accurate particle diameter measurement even with respect to one and the same particle.